ProjectSummary Telomeres and rDNA are each crucial for the basic cellular functions of chromosome end protection and ribosome biogenesis, respectively. At the same time, they each constitute major repetitive domains of the genome that can contribute to genomic instability. Repetitive sequences pose challenges to the replication machinery that can lead to fork stalling and DNA damage. Damage can be repaired by homology directed recombination and repair using the sister chromatid following replication. Cohesion between sister chromatids isessentialforthisrepair.Cohesionisparticularlyimportantforrepetitivesequences,tokeepthemalignedand topreventrecombinationwithnon-sisterchromatids.Whileitiswellestablishedthatcohesinringsarerequired toholdsisterchromatidstogetheratthemacrolevel,itisnotclearhowthemoreintimatecontacts,particularly those required for repetitive DNA, are mediated. Our published work and preliminary data in this proposal indicatethatthesetwoessentialrepetitivedomains(telomeresandrDNA)mayhaveacommonmechanismfor controlling sister chromatid cohesion that is distinct from the rest of the genome. This research project builds on our recent discovery that a) the cohesin ring plays a limited role in telomere cohesion and thus novel proteins and mechanisms are involved and b) rDNA cohesion is governed by the same mechanism as telomeres.InthefirstpartofthegrantinAim1wewillcontinueourinvestigationoftelomerecohesionandgo deeperintothemechanismsusedbytheproteinsthatwehaveshownarerequiredandidentifynewproteins. In the second part of the grant in Aim 2 we will break new ground by investigating cohesion of the rDNA arrays. Together these studies will provide insight into how these essential repetitive domains manage the intimatemolecularconnectionsthatarerequiredforgenomeintegrityandhowtheirdysfunctioncontributesto genomeinstabilityandpathology.